1. Field of the Invention
The present invention relates to RFID (Radio Frequency Identification) positioning system, and more particularly, to a system and method for positioning an active RFID tag.
2. Related Art
RFID (Radio Frequency Identification) system has two basic elements: RFID tag and RFID reader. RFID tag may have an IC (Integrated Circuit) to process and modulate (Radio Frequency) RF signals. RFID tag may be classified into active and passive tags according to if an independent power source is embedded therein. An active RFID tag has an independent power source to provide necessary electricity to the IC, while a passive RFID tag generates sensing current through magnetic induction to conduct the operation of the IC. When the RFID reader receives the RF signal from the active RFID tag, the RFID reader may estimate the distance between the active RFID tag and the RFID reader according to the changes of signal intensity (Radio Signal Strength Indicator, RSSI) of the RF signal. Based on such characteristic, the active RFID tag is generally used for the positioning purpose.
Triangulation is commonly used to position a signal source. In the so-called triangulation, as shown in FIG. 1A, at least three RFID readers 101˜103 are used to trace the signal intensity of the same signal source 104a. The possible distances 105˜107 between the signal source 104a and each of the RFID readers 101˜103 are calculated first. Then use the RFID readers 101˜103 that are able to receive RF signals from the signal source 104a as a circle center, so as to calculate the overlapped area 108 of the possible ranges of signal source 104a and the RFID readers 101˜103. The obtained overlapped area 108 indicates the possible location of the signal source 104a. The triangulation basically uses single-time positioning approach to position the location of the signal source.
The triangulation in the prior art is also applied to position the location of the active RFID tag. However, in the circumstance of interior positioning, the signal intensity is easily changed. If the signal intensity received directly is used to estimate the possible location and obtain the overlapped area 108, a greater distance difference may thereby be generated. As shown in FIG. 1B, assume there is a barrier 109 disposed between the signal source 104b and the RFID reader 101. The signal intensity received by the RFID reader 101 is possibly much weaker, which causes misjudgments of the distance 105 between the signal source 104b and the RFID reader 101 to be longer than the actual distance 110. If such conditions happens on other RFID readers (such as 102 and 103) at the same time, the possible area 108 obtained from the single-time positioning of the received signal intensities of each of the RFID readers 101˜103 may possibly deviates from the actual location of the signal source 104b. 
Therefore, in the prior art the interior single-time positioning from the RF signals received by the RFID reader is not precise due to the interior signal intensity of the RFID reader has unstable changes.